fix(chapter3): correct links to future chapters
fix(chapter3): correct links to future chapters

cjmazey/fork_leanprover_tutorial,avigad/tutorial,Hodapp87/cs6460_project,leanprover/tutorial,fpvandoorn/tutorial,fpvandoorn/tutorial,Hodapp87/cs6460_project,cjmazey/fork_leanprover_tutorial,cjmazey/fork_leanprover_tutorial,fpvandoorn/tutorial,Hodapp87/cs6460_project,avigad/tutorial,soonhokong/lean-tutorial,leanprover/tutorial,soonhokong/lean-tutorial,avigad/tutorial,soonhokong/lean-tutorial,leanprover/tutorial